The present invention is directed to a picture tube screen device having a waiting state function.
Picture tube screen devices such as, for example, video display terminals, are often turned on for an entire work day or, for example, in a computer center, even during an entire night, although the actual time of use amounts to only a fraction of the on time. The overall, useful operating life of a picture tube screen device is essentially limited by the life expectancy of the picture tube. The life expectancy of the picture tube is defined by the effectiveness of the heated cathode and the effectiveness of the heated cathode decreases over the course of time. In quality products, the useful life of the picture tube is approximately 10,000 hours. Due to the decrease in the effectiveness of the cathode, the brightness of the picture tube drops to 70% of the original value after the 10,000 hours.
Several solutions have been disclosed for preserving the picture tube. The picture tube screen device is brought into a waiting state or in what is referred to as a "stand-by mode". In a first solution, the monitor part is shut off and thus all voltages at the picture tube are also shut off. The disadvantage is that a heating-up phase must be passed through for every re-utilization. In a second solution, the monitor part is effectively shut off, but the heating of the picture tube is not shut off. In this way, an image, what is referred to as an immediate image, very quickly appears following a keyboard input at a video display terminal. A third solution for bringing the picture tube screen device into a waiting state is to only disconnect the video signal and thereby causing the picture tube to be dark. As a result, the picture tube continues to be heated and the grid voltages continue to be applied at the picture tube. However, none of these solutions represent favorable operating conditions for the purpose of preserving the picture tube and, thus, extending the operation of the picture tube.
A continuous heating of the picture tube without applied grid voltages or without beam current can lead to what is referred to as "cathode poisoning" that is a chemical modification of the cathode substance due to foreign atoms that leads to a reduction in the obtainable image brightness. Given frequent turning on and off of the picture tube heating, the picture tube cathode is highly stressed by the high currents at turn on and thermal load cycles.